1. Field of the Invention
The present invention involves the blood-brain barrier and methods for determining the degree of transcytosis of peptides or plasma proteins across the blood-brain barrier.
2. Description of Related Art
The vertebrate brain has a unique capillary system which is unlike that in any other organ in the body. The unique capillary system has morphologic characteristics which make up the blood-brain barrier (BBB). The blood-brain barrier acts as a systemwide cellular membrane which separates the brain interstitial space from the blood.
The unique morphologic characteristics of the brain capillaries which make up the BBB are: (a) epitheliallike high resistance tight junctions which literally cement all endothelia of brain capillaries together, and (b) scanty pinocytosis or transendothelial channels, which are abundant in endothelia of peripheral organs. Due to the unique characteristics of the blood-brain barrier, hydrophilic drugs and peptides that readily gain access to other tissues in the body are barred from entry into the brain or their rates of entry are very low.
Various strategies have been developed for introducing such hydrophilic drugs and peptides into the brain, which otherwise would not cross the blood-brain barrier. The most widely used strategies involve invasive procedures where the drug is delivered directly into the brain. The most common procedure is the implantation of a catheter into the ventricular system to bypass the blood-brain barrier and deliver the drug directly to the brain.
Recently, techniques have been discovered for altering hydrophilic drugs and peptides so that they can be delivered to the brain by receptor-mediated transcytosis through the blood-brain barrier. For example, chimeric peptides have been prepared which include a peptide, which by itself, is capable of crossing the blood-brain barrier by transcytosis at a relatively high rate. This transportable peptide is conjugated to a hydrophilic neuropeptide which, by itself, is transportable only at a very low rate into the brain across the blood-brain barrier. The resulting chimeric peptide is transported into the brain at a much higher rater than the neuropeptide alone. Details of such chimeric peptides are set forth in U.S. patent application Ser. No. 06/892,067, filed on July 30, 1986.
Other techniques for increasing transcytosis of hydrophilic drugs and peptides across the blood-brain barrier include cationization of antibodies used in treatment and diagnosis of neurological diseases. It has been found that cationization of antibodies renders them transportable through the blood-brain barrier by transcytosis. Details of cationized antibodies for delivery through the blood-brain barrier are set forth in U.S. patent application Ser. No. 07/085,627, filed on Aug. 17, 1987.
In view of the development of substances that are capable of crossing the blood-brain barrier by transcytosis, there is presently a need to provide a routine and quantitative methodology for rapidly accessing the degree of transport by transcytosis through the cellular endothelial barrier in the brain capillaries. Presently, morphologic methods are available, such as autoradiography or immunocytochemistry. Both of these methods involve exposing the brain to various compounds followed by sectioning of the brain and measurement of the compound distribution.
The autoradiographic method, although sensitive, is very slow. This method requires several months, delay for adequate exposure of the emulsions so that proper assessment of radio-activity in the brain tissue can be made. Immunocytochemistry is more rapid than the autoradiographic method. However, the immunocytochemistry method is very insensitive and therefore, is unsuitable for use in detecting minute amounts of material which may be transported across the blood-brain barrier.
As is apparent, there is a need to provide a simple, efficient and quantitative method for measuring the transcytosis of compounds through the blood-brain barrier so that an accurate assessment can be made regarding the compound's usefulness in the diagnosis or treatment of neurological diseases.